In centrifugal separators of the above-noted type, known as nozzle centrifuges, the separated underflow is discharged through nozzle means arranged at the outer periphery of the separating chamber in the centrifugal bowl. In the use of such centrifuges, it is often necessary to control the solids content of the discharging underflow by recycling part of it to the centrifuge inlet. The most common application of underflow recycling is in cases where the feed to the centrifuge has a low content of solids, and the desired result is a high concentration of solids in the underflow. The need for adequate control in these cases is defined by two extremes in which the control is inadequate or non-existent, namely, (1) the underflow contains too much feed liquid or (2) it contains too high a concentration of solids so as to cause plugging of the discharge nozzles.
It is general knowledge in the art that for most liquids an increase in suspended solids results in an increased viscosity of the liquid. Consequently, an ideal way to control the solids content of the underflow would be to monitor and control the viscosity of the underflow.
Many systems have been proposed for controlling the solid content of a centrifuge underflow. However, these prior systems are quite insensitive to changes in the viscosity of the underflow. Viscosity is defined as the ratio of shear stress to shear rate. More simply, viscosity is the inherent property of a liquid to resist deformation from shear. For liquid flow, viscosity can be measured as a change in velocity of a moving liquid due to an applied shear force. For example, the shear stress exerted at the wall of a pipe on a liquid flowing through it results in a net loss in velocity of the liquid. Since stress is a measurement of force per unit area, an increase in shear area will result in an increase in shear stress created within the liquid. In the above example, an increase in pipe length will increase the shear area with a net loss in liquid velocity for a constant pressure drop.
Most systems for control of recycled underflow have relied upon valves and/or orifices. These have been used as flow restrictions in various combinations in the underflow return path, in the flow stream of the non-returned part of the underflow (reject) or a combination of both streams.
The problem with using a conventional valve or orifice to control underflow concentration lies in the fact that these flow restrictors are typically insensitive to viscosity. The pressure drop across a valve or orifice is a function of the velocity and viscosity of the liquid flowing therethrough. Since the length of the valve or orifice in the direction of the liquid flow is small, the ratio of shear area to cross-sectional area of the valve opening or orifice is small. There is a minimal shear area, therefore, in the valve or orifice, which means that that the pressure drop across it is primarily a function of velocity. Viscosity has only a small effect on pressure drop, so that viscosity changes have only a minimal effect on the pressure drop. Since mass flow through typical valves and orifices is primarily a function of pressure drop, those devices are not suited for controlling the centrifuge underflow. In fact, most of the pressure drop due to viscosity in prior recycle systems is due to the connecting pipework and not the valves or orifices.
An example of a system for controlling the underflow from nozzle centrifuges is disclosed in U.S. Pat. No. 4,162,760 issued to Hill in 1979. That prior system uses an adjustable head sump for recycling with an adjustable toroidal ring-type valve. Because of its limited shear area, the valve is no more viscosity-sensitive than a needle valve. As the valve is opened to allow the desired flow therethrough, the ratio of its shear area to its cross-sectional flow area is drastically decreased, resulting in loss of sensitivity to viscosity and viscosity changes. There is no way in which the sensitivity to viscosity can remain constant for selected changes in recycle solids concentration or recycle rate with this type of arrangement.
As the underflow solids content increases, the point at which the centrifuge will plug is approached. Therefore, it is desirable to have the greatest sensitivity to viscosity, and hence to solids content at the higher solids concentrations. However, the converse is true with the arrangement in U.S. Pat. No. 4,162,760. In order to increase the solids concentration in the underflow, the toroidal ring must be opened to allow a greater volume of thickened solids to recycle through the centrifuge. As the ring is opened, the ratio of shear area to cross-sectional flow area is reduced, thereby reducing the sensitivity to viscosity and ultimately to underflow solids concentration. Thus, the point at which the need for sensitivity to viscosity is the most critical is the point at which the arrangement in U.S. Pat. No. 4,162,760 is the lease sensitive to viscosity.